This expression is from a book Eat, Pray,Love by a writer Elizabeth Gilbert.
I try to guess the meaning but still don't know.
Below is the situation the writer is standing at her house's door with her friend, expecting him to kiss her.
This is an improvement; for the first few weeks, he would only shake my hand. I think if I were to stay in Italy for another three years, he might actually get up the juice and kiss me.
Best Answer
I think get up the juice in this context (he might actually get up the juice and kiss me) alludes to a more common idiom: get up the courage/nerve, and this juice is used in the sense of "courage" or "nerve"; so the whole clause would mean "he might actually get enough courage and kiss me".
Related definitions given by The Free Dictionary: